1. Field of the Invention
The present invention relates to compositions incorporating urethane for use in sporting equipment. In particular, the invention relates to such compositions for use in golf ball cores, inner covers, outer covers, and intermediate layers. The present invention also relates to methods of manufacture of these compositions.
2. Description of Related Art
Sports equipment often incorporates polymeric materials. These materials are chosen because they provide good properties with respect to cost, weight, and durability in a variety of uses. In particular, polymers are used in the manufacture of golf balls. Golf balls generally include a core and at least one cover layer surrounding the core. Balls can be classified as two-piece, multi-layer, or wound balls. Two-piece balls include a spherical inner core and an outer cover layer. Multi-layer balls include a core, a cover layer and one or more intermediate (or mantle) layers. The intermediate layers themselves may include multiple layers. Wound balls include a core, a rubber thread wound under tension around the core to a desired diameter, and a cover layer, typically of balata material.
Material characteristics of the compositions used in sports equipment, including golf ball layers are important in determining the durability and performance of the equipment. For example, with respect to golf balls, the composition of a golf ball cover layer is important in determining the ball's durability, scuff resistance, speed, shear resistance, spin rate, feel, and “click” (the sound made when a golf club head strikes the ball). Various materials having different physical properties are used to make cover layers to create a ball having the most desirable performance possible. For example, many modern cover layers are made using soft or hard ionomer resins, elastomeric resins or blends of these. Ionomeric resins used generally are copolymers of an olefin and the metal salt of an unsaturated carboxylic acid(s), or are ionomeric terpolymers having at least one additional monomer polymerized into its structure. These resins vary in resiliency, flexural modulus, and hardness. Examples of these resins include those marketed under the tradenames SURLYN (E.I. du Pont de Nemours & Company, Wilmington, Del.) and IOTEK (ExxonMobil Corporation, Irving, Tex.).
Elastomeric resins used in golf ball covers include a variety of available thermoplastic or thermoset elastomers. Balata and thermoplastic and thermoset polyurethane are the three most commonly used materials in this category.
Layers other than cover layers also significantly affect performance of a ball. The composition of an intermediate layer is important in determining the ball's spin rate, speed, and durability. The composition and resulting mechanical properties of the core are important in determining the ball's coefficient of restitution (C.O.R.), which affects ball speed and distance when hit. In addition to the performance factors discussed above, processability also is considered when selecting a formulation for a golf ball composition. Good processability allows for ease of manufacture using a variety of methods known for making golf ball layers, while poor processability may lead to avoidance of use of particular materials, even when those materials provide for good mechanical properties. These same considerations of durability and ease of manufacture are relevant for a wide variety of sports equipment.
Various materials having different physical properties are used to make sports equipment having the most desirable performance possible. One material generally cannot optimize all of the important properties for a particular piece of equipment. For golf balls, properties such as feel, speed, spin rate, resilience and durability all are important, but improvement of one of these properties by use of a particular material often may lead to worsening of another. For example, ideally, a golf ball cover should have good feel and controllability, without sacrificing ball speed, distance, or durability. Despite the broad use of copolymeric ionomers in golf balls, their use alone in, for example, a ball cover may be unsatisfactory. A cover providing good durability, controllability, and feel would be difficult to make using only a copolymeric ionomer resin having a high flexural modulus, because the resulting cover, while having good distance and durability, also will have poor feel and low spin rate, leading to reduced controllability of the ball. Also, the use of particular elastomeric resins alone may lead to compositions having unsatisfactory properties, such as poor durability and low ball speed.
Therefore, to improve the properties of sports equipment produced from polymers, the polymer materials discussed above may be blended to produce improved equipment parts. For example, compositions for use in golf balls have involved blending high-modulus copolymeric ionomer with lower-modulus copolymeric ionomer, terpolymeric ionomer, or elastomer. As discussed above, ideally a golf ball cover should provide good feel and controllability, without sacrificing the ball's distance and durability. Therefore, a copolymeric ionomer having a high flexural modulus often is combined in a cover composition with a terpolymeric ionomer or an elastomer having a low flexural modulus. The resulting intermediate-modulus blend possesses a good combination of hardness, spin and durability.
Sports equipment prepared from polymer often is prepared using one of three known methods of manufacture: casting, injection molding, or compression molding. Of the three methods, injection molding generally is preferred, due to the efficiencies gained by its use. Injection molding generally involves using a mold having one or more sets of two mold sections that mate to form a cavity in the shape of the intended part during the molding process. For example, in forming a golf ball layer over a core, the pairs of mold sections are configured to define a spherical cavity in their interior when mated. When used to mold an outer cover layer for a golf ball, the mold sections may be configured so that the inner surfaces that mate to form the spherical cavity include protrusions configured to form dimples on the outer surface of the molded cover layer. The mold sections are connected to openings, or gates, evenly distributed near or around the parting line, of the mold sections through which the material to be molded flows into the cavity. The gates are connected to a runner and a sprue that serve to channel the molding material through the gates. When used to mold a layer onto an existing structure, such as a ball core, the mold includes a number of support pins disposed throughout the mold sections. The support pins are configured to be retractable, moving into and out of the cavity perpendicular to the spherical cavity surface. The support pins maintain the position of the core while the molten material flows through the gates into the cavity between the core and the mold sections. The mold itself may be a cold mold or a heated mold. In the case of a heated mold, thermal energy is applied to the material in the mold so that a chemical reaction may take place in the material.
Because thermoset materials have desirable mechanical properties, use of such materials in sports equipment generally is desirable. Unfortunately, thermoset materials generally are not well suited for injection molding, because as the reactants for thermoset polyurethane are mixed, they begin to cure and become highly viscous while traveling through the sprue and into the runners of the injection mold, leading to injection difficulties. For this reason, thermoset materials typically are formed into sports equipment using a casting process free of any injection molding steps.
In contrast to injection molding, which generally is used to prepare layers from thermoplastic materials, casting often is used to prepare parts from thermoset material (i.e., materials that cure irreversibly). In an example casting process for making a golf ball layer over a core, the thermoset material is added directly to the mold sections immediately after it is created. Then, the material is allowed to partially cure to a gelatinous state, so that it will support the weight of a core. Once cured to this state, the core is positioned in one of the mold sections, and the two mold sections are then mated. The material then cures to completion, forming a layer around the core. The timing of the positioning of the core is crucial for forming a layer having uniform thickness. The equipment used for this positioning are costly, because the core must be centered in the material in its gelatinous state, and at least one of the mold sections, after having material positioned therein, must be turned over and positioned onto its corresponding mold section. Casting processes often lead to air pockets and voids in the layer being formed, resulting in a high incidence of rejected golf balls. The cost of rejected parts, complex equipment, and the exacting nature of the process combine to make casting a costly process in relation to injection molding.
Compression molding also is used for making parts for use in sports equipment, and it often is combined with injection molding. For example, compression molding of a golf ball layer typically requires the initial step of making half shells by injection molding the layer material into a cold injection mold. The half shells then are positioned in a compression mold, whereupon heat and pressure are used to mold the desired part. Compression molding also may be used as a curing step after injection molding. In such a process, thermally curable material is injection molded around in a cold mold to create a part. After the material solidifies, the part is removed and placed into a mold, in which heat and pressure are applied to induce curing in the part.
As mentioned above, one material used in sports equipment is polyurethane. Polyurethane typically is formed as the reaction product of a diol or polyol, along with an isocyanate. The reaction also may incorporate a chain extender configured to harden the polyurethane formed by the reaction. Thermoplastic polyurethanes have generally linear molecular structures and incorporate physical cross-linking that may be reversibly broken at elevated temperatures. As a result, thermoplastic polyurethanes may be made to flow readily, as is required for injection molding processes. In contrast, thermoset polyurethanes have generally networked structure that incorporate irreversible chemical cross-linking. As a result, thermoset polyurethanes do not flow freely, even when heated.
Thermoplastic and thermoset polyurethanes both have been used in, for example, golf ball layers, and each provides for certain advantages. Because of their excellent flowability, thermoplastic polyurethanes may be positioned readily around a golf ball core using injection molding. Unfortunately, parts comprising thermoplastic polyurethane exhibit poor durability; for example, golf balls from thermoplastic polyurethane exhibit poor shear-cut resistance. Thus, while thermoplastic polyurethane parts are less expensive to make due to their superior processability, they are not favored due to the resulting inferior performance. In contrast, thermoset polyurethane exhibits high shear-cut resistance and is much more scuff- and cut-resistant than thermoplastic polyurethane. However, the irreversible cross-links in the thermoset polyurethane structure make it unsuitable for use in injection molding processes conventionally used for thermoplastic materials.
Thermoplastic polyurethanes are used in sports equipment. Examples of their use in golf ball compositions are discussed in U.S. Pat. No. 5,759,676 to Wu, which discloses thermoplastic polyurethane utilized in blends for mantle and cover layers, and in U.S. Pat. No. 6,319,152 to Takesue, which teaches blending of a thermoplastic polyurethane with a styrene-based block copolymer to increase the scuff resistance of the resulting golf ball cover. The Takesue patent discloses that because thermoplastic polyurethanes are “inexpensive and easy to mold, these elastomers are regarded as an excellent cover stock substitute for balata material. However, the thermoplastic polyurethane elastomers are still insufficient in scuff resistance upon iron shots.” Thermoplastic polyurethanes also are used for making mantle layers to give the feel of a wound ball to non-wound constructions. Such a mantle is disclosed in U.S. Pat. No. 5,759,676 to Cavallaro et al.
Though they are more expensive to process than thermoplastic polyurethanes, thermoset polyurethanes also have been used in golf ball layers. For example, U.S. Pat. No. 6,132,324 to Hubert discloses a golf ball having a cover formed from thermoset polyurethane. The patent teaches a method for casting a thermoset polyurethane cover over an ionomer inner layer, including a step of measuring the viscosity “over time, so that the subsequent steps of filling each mold half, introducing the core into one half and closing the mold may be properly timed for accomplishing centering of the core cover halves fusion and overall uniformity.” The additional steps involved in casting a layer over those needed for injection molding the layer lead to added complexity and expense. Another patent discussing use of thermoset polyurethane is U.S. Pat. No. 6,435,987 to Dewanjee. This patent teaches thermosetting polyurethane comprising a toluene diisocyanate-based prepolymer, a second diisocyanate prepolymer, and a curing agent. Again, this method makes use of casting because the materials used would not be well suited to injection molding. One attempt to successfully use thermoplastic polyurethane in golf ball covers is disclosed in U.S. Pat. No. 6,123,628 to Ichikawa et al. This patent discloses golf ball covers incorporating the reaction product of a thermoplastic polyurethane with an isocyanate compound. In this patent, the cross-linking reaction is completed during extrusion. The completed golf ball covers are thermoplastic, and they provide for improved scuff resistance, though they do not exhibit improvements in other mechanical properties.
In view of the above, it is apparent that polymer parts for sports equipment that provide optimal performance and durability properties, while demonstrating ease of manufacture, as well as methods for making these parts, are needed. The present invention fulfills this need and provides further related advantages.